Add exception handling for `libcpp::stack::push`

Since cimporting directly from "cpython" has long been deprecated.

e.g. https://github.com/cython/cython/issues/4707

Acquire the GIL in nogil functions only when strictly needed on function exit, e.g. for cleaning up temp variables from with-gil blocks or adding tracebacks.

Closes GH-3554
Closes GH-4637

The license follows that of the official Python logo, which it is derived from.

Unconditionally adding `/D_USE_MATH_DEFINES` via cmdline triggers `-Wmacro-redefined` in cython-generated code.

Unconditionally adding `/D_USE_MATH_DEFINES` via cmdline triggers `-Wmacro-redefined` in cython-generated code.

It appears to have been be used for Python < 2.6 (which didn't have the new buffer protocol).
Most of the mechanism had already been removed, but the capsule attributes that it relied on were still generated.

Also removed some utility code that was only used for this feature.

For fused functions it makes sense that the return type is ignored
(a function can't be specialized based on return type alone) but
for function pointers the return type should be included (since
such a pointer might be an argument to a fused function)

Fixes https://github.com/cython/cython/issues/4644

Make the pickle test actually test that auto-pickling uses the right checksum (and not a user implemented reduce method that uses no checksums at all).

Make error message more readable by using hex integer values for all presented checksums, not just the compile time ones.

For the auto-pickle checksum, allow SHA-1 and SHA-256 which are used by Cython 3.x pickles now. Otherwise stick to MD5 since that was used before.

Closes https://github.com/cython/cython/issues/4680

For the auto-pickle checksum, allow MD5 again since it was used by Cython 0.x pickles. Otherwise switch to SHA-256 since that should be safe enough to stay available in Python's hashlib module for another while.

Closes https://github.com/cython/cython/issues/4680

In https://github.com/python/cpython/pull/31530
https://bugs.python.org/issue46836

the `_frame` struct was moved to an internal header, however the public API is
primarily read-only, and Cython needs to build PyFrameObjects so still import
the internal headers.

Also sets the Py_BUILD_CORE define for py311a6, trying to restrict it to the frame header.

In https://github.com/python/cpython/pull/31530
https://bugs.python.org/issue46836

the `_frame` struct was moved to an internal header, however the public API is
primarily read-only, and Cython needs to build PyFrameObjects so still import
the internal headers.

Also sets the Py_BUILD_CORE define for py311a6, trying to restrict it to the frame header.

Closes https://github.com/cython/cython/issues/3066

Fix minimal compile mode. FlowControl.py also needs to be compiled since it depends on the C level interface of Visitor.py.

Memoryview temps weren't being properly incremented (at the start
of the loop) or decremented (at the end of the loop). They were,
however, decremented in the exception handling case.

Long-term I think we really should unify reference counting for
memoryviews further. It's *slightly* different from PyObjects
(mainly that it happens in wrapper functions rather than the
main function) and that leads to a large chance that it just
gets missed.

Fixes https://github.com/cython/cython/issues/4662

Improve the bug/feature templates to provide a clearer hint why cython-users is a better place to get help than the bug tracker.

Fix test by adding "expected" warnings. These should rather be avoided, but this is the easiest fix for now (and it's not the only ones).

See https://github.com/cython/cython/issues/4310

Turn the redeclaration of attributes as C methods into an error since it currently leads to difficult to predict behaviour. (#4661)